A storage system comprising a first storage device and a second storage device that have different access speeds has recently been developed. Assume here that the access speed and storage capacity of the first storage device are high and small, respectively, and that the access speed and storage capacity of the second storage device are lower and greater than those of the first storage device, respectively. The storage system is realized by hierarchically combining the first and second storage devices as upper and lower tiers, respectively. This technique is also called a storage tiering technique, and the storage system employing the storage tiering technique is also called a tiered storage system.
In the tiered storage system, tiers (storage devices) in which data is to be arranged (stored) are determined/changed in accordance with the characteristics of the data. More specifically, data having a high access frequency is arranged in the upper tier (first storage device), and data having a low access frequency is arranged in the lower tier (second storage device). This arrangement (namely, tiering) enables the tiered storage system to realize both high performance and low cost.
In general, tiering in the tiered storage system is automatically performed. For the automatic tiering, a storage controller monitors, for example, an access status in the tiered storage system data group by data group called an extent. The extent generally has a size greater than the basic size (so-called block size) for data management (data access). The status of access to each extent is expressed by an access statistical value, such as the number of accesses.
The storage controller performs, for example, periodically, estimation associated with the access frequency of each extent, based on the access statistical value thereof. Based on the periodical estimation result, the storage controller transfers (rearranges) data from the lower tier to the upper tier and/or vice versa extent by extent. However, data transfer between the tiers based on the periodical access frequency estimation will cause significant overhead and require a large amount of time.